The present invention relates to the perfume industry. More particularly, it concerns a new process for the preparation of 4-tert-butyl-cyclohexanol essentially in the form of its cis isomer. This compound is useful in the fragrance industry, particularly as an intermediate product for the preparation of esters, namely acetic ester.
Specifically, 4-tert-butyl-cyclohexyl acetate is a well-known perfuming ingredient, which is available on the market under several tradenames. It is generally commercialized in the form of an isomeric mixture wherein the cyclanic cis isomer is present in varied proportions, more often in an amount between 30 and 65% by weight. Now, it has been established that there is a fundamental difference between the olfactive quality of this cis isomer and that of the corresponding trans isomer, the first of these compounds being preferred [see Arctander, Perfume and Flavor Chemicals, Montclair, N.J. (1969), sects. 440 and 441]. For this reason, a great number of research groups have directed their efforts towards the preparation of said ester in its preferred isomeric form. Nevertheless, the problem of the availability of a product rich in the cis isomer remains of actuality. As a matter of fact, while some of the numerous syntheses disclosed in the prior art provide isomeric mixtures with an adequate content in cis isomer, this is only achieved at the cost of expensive technical solutions which render their industrial exploitation not very profitable.
U.S. Pat. No. 2,927,127, granted on Mar. 1, 1960, describes a process for the preparation of a cis/trans mixture of 4-tert-butyl-cyclohexyl acetate having a content in cis isomer as high as 87.5%, which process consists in the catalytic hydrogenation of p-tert-butyl-phenol in the presence of Rh on an active charcoal support and in the acetylation, by means of acetic anhydride, of the product thus obtained. The hydrogenation step is carried out at a high pressure, of the order of 7.times.10.sup.6 Pa.
Likewise, in Chemical Abstracts, vol. 80, sec. 14700s, there is disclosed a process for reducing p-tert-butyl-phenol by means of a catalytic hydrogenation with Rh on active charcoal or with ruthenium dioxide, at a pressure of 10.times.10.sup.6 Pa. The reported content in cis isomer was 64.1%.
U.S. Pat. No. 4,343,955 teaches a process for the preparation of p-tert-butyl-cyclohexanol by catalytic hydrogenation in the presence of ruthenium on alumina, at a pressure of 4.times.10.sup.6 Pa and a temperature of 100.degree. C. In this case, the content of the resulting product in cis isomer was 74.8%.
Finally, European Patent no. 141 054 describes a two-step process comprising the reduction of p-tert-butyl-phenol by means of palladium on alumina, at 120.degree.-180.degree. C. and a pressure above 20.times.10.sup.6 Pa, followed by the hydrogenation of the resulting product in the presence of ruthenium on same alumina support, at a temperature of 70.degree.-200.degree. C. and under the same pressure conditions. The cis isomer was present in the obtained mixture in an amount of the order of 46%.
As described, the prior art processes resorted to methods requesting drastic reaction conditions, thus creating technical problems of a particular nature when such methods are applied at an industrial scale. On the other hand, it is well-known that amongst the questions to be considered when applying industrially a process which uses heterogeneous catalysis, lies that of the feasibility of recycling the catalyst, as well as the concern for its lifetime and reactivation. All these factors enter into account when determining the worth of a process from an economical point of view and are, as a result, critical factors when said process is put into industrial application.
It has now been discovered that, henceforth, it is possible to obtain isomeric mixtures of 4-tert-butyl-cyclohexanol which are rich in the cis isomer, through a process that, in addition, presents clear advantages over the prior art methods in what concerns the recycling of the catalyst. Although this process comprises the catalytic hydrogenation of 4-tert-butyl-phenol or of 4-tert-butyl-cyclohexanone in the presence of a rhodium-based catalyst, it allows nevertheless the application of mild reaction conditions. It has in fact been observed that the use of a catalyst composed of rhodium previously deposited on an alumina support in the presence of a co-catalyst such as boron trifluoride, not only made it possible to obtain an excellent stereoselectivity, but also provided a catalytic system the activity of which remained constant and adequate over a high number of operations, thus rendering the process more cost-effective. While the use of fluorinated catalysts of the type BF.sub.3.Al.sub.2 O.sub.3 has been described in the context of alkylation reactions involving aromatic systems and KF.Al.sub.2 O.sub.3 has been reported as a catalyst in simple addition-type reactions [see J. Chem. Soc. 1986, 1133-39] , there are few reports of the use of such catalysts in hydrogenation reactions and, to the best of our knowledge, their use in the stereoselective hydrogenation of cyclohexanones or phenols, to yield cyclohexanols, has never been suggested. As a co-catalyst, BF.sub.3 may be employed as such or in the form of one of its complexes.